Radio-frequency (RF) ranging technology provides information regarding distance and relative position between objects. An RF ranging system calculates the distance between two objects based on the time a radio signal propagates between the two objects and the speed of the radio signal. In air, the radio signal propagates at a constant rate, roughly equal to the speed of light.
Traditionally, the RF ranging system utilizes highly accurate, synchronized clocks to calculate the propagation time. For example, in such a system, a first radio device and a second radio device both contain clocks synchronized such that the second radio device receives a signal from the first radio device and compares a signal departure time to a signal arrival time to calculate the propagation time. The system multiplies this propagation time by the speed of light to estimate a distance between the first and second radio devices. Maintaining a system of clocks synchronized to the level of accuracy required to make this type of system practicable for ranging purposes represents a significant drawback.
Early approaches to RF ranging systems were primarily dominated by continuous wave (CW) and other narrow bandwidth systems. While CW systems, such as tellurometers, enable long distance ranging in the tens of kilometers and accuracies of 1 cm at a distance of 1 km in low multipath environments, these systems often suffer from poor multipath performance and susceptibility to jamming, and tend to cause interference to other communications systems. In recent years, CW systems have mostly been superseded by ultra-wideband (UWB) and various spread spectrum systems.
UWB systems in the U.S. are constrained to operate within Federal Communications Commission (FCC) regulations. Thus, their range is typically limited to a few hundred meters using directional antennas in low multipath environments, and typically much less than 100 meters indoors with omni-directional antennas. UWB systems also require complex RF electronics which can drive up system cost and raise regulatory issues.
RF ranging technology may also be used to implement a real-time locating system. For example, a first radio device in the real-time locating system may be attached to an object, such as a vehicle, for which a location is to be determined. Other radio devices in the real-time locating system may each use the RF ranging technology to calculate a distance from the first radio device. The location of the object may then be determined based on the calculated distances and the locations of those other radio devices.
An example of such real-time locating system is a global positioning system (GPS), which provides an absolute location for an object. In many situations, however, GPS signals may be either unavailable or actively denied to a potential user. Thus, there is a need for real-time locating systems and methods to address the issues described above.